When you tuck into a tasty salmon steak, remember that you’re most likely enjoying the product of intensive aquaculture. Around 95 per cent of all adult Atlantic salmon in existence are farmed, and domestication has made them very different to wild populations, each one of which is adapted to its own river system.

Although farmed salmon are genetically different to their wild counterparts, it turns out they are just as fertile. This is important because millions of farmed salmon escape into the wild. Here they pose a threat to the gene pool of wild salmon, if they can successfully cross-breed.

Farmed salmon grow very fast, are aggressive feeders and not as clever as wild salmon when it comes to dealing with predators. These traits are good for producing fish for the table, but not for the stability of wild populations, each of which has its own locally adapted strategy for growth, maturity, timing its migration and resisting disease.

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Swamping wild stocks

The problem is that by escaping, farmed salmon could introduce negative traits into wild spawning populations. The sheer scale of salmon farming and potential escapes means that the farmed fish could swamp already-depleted wild populations – down to a tenth of their numbers of 50 years ago – eroding and diluting locally adapted wild gene pools.

So why the call for action now? Until recently, there was no detailed data on how the sperm and eggs of domesticated salmon fared compared to their wild counterparts. People suspected farmed salmon fared less well because of selective breeding or relaxed selection under domestication, and therefore the threat of cross-breeding was reduced.

Past tests on adult salmon straight from the farm found that they have reproductive potential, but were generally inferior in spawning because of inappropriate behaviours and especially poor male competitiveness. However, newly published research that I headed shows that farmed fish are just as potent at the gamete level as wild fish (Evolutionary Applications, DOI&colon; 10.1111/eva.12148), and if farm escapees can revive their spawning behaviour in the wild, they present a clear threat of hybridisation with wild populations.

In vitro testing

We tested gamete performance using a range of in vitro fertilisation experiments in conditions that mimicked natural spawnings. Our tests looked at sperm and egg form and function, fertility (under demanding conditions), sperm competitiveness, and sperm-egg compatibility. All our results showed that farmed salmon gametes are just as fertile and competitive as wild salmon.

There is ample evidence that escaped farmed salmon can survive at sea and get into spawning rivers. In some Norwegian rivers, big numbers of farmed fish have been recorded – accounting for as much as half of the salmon. There is also evidence that farmed fish have successfully mated with wild populations&colon; the genetic signatures of salmon in some Norwegian rivers now exhibit significant changes that are entirely consistent with wild/farmed hybridisation.

Salmon farming is a huge and successful business in the UK, Norway and beyond, and although it does reduce pressure on wild fish stocks, it can also create its own environmental pressures. The genetic disruption of wild fish is one example.

Make them triploid

So what can be done? A solution is to induce triploidy – cells with three sets of chromosomes – by pressure-treating salmon eggs just after fertilisation. Triploidy has been applied to rainbow trout farming in the UK for decades, and certain food crops like bananas and some apples are triploid. Importantly, triploid salmon are almost all sterile.

Although major triploid trials are ongoing in Norwegian farms, with results due out next year, some parts of the aquaculture industry seem to be resisting this technology, because of fears that triploids don’t grow or perform as well in farms as normal diploid fish and might even be non-viable commercially. However, recent tests indicate that triploids can perform as well as diploid farmed fish, possibly even better with careful selective breeding.

Now that our research has further highlighted a reproductive threat from escaped farm salmon, perhaps it’s time to put serious effort into developing triploidy as standard practice for salmon aquaculture, so that one of the many pressures facing wild Atlantic salmon stocks can be removed.